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71 CHAPTER FIVE CONSTRUCTION AND FIELD TESTING SCOPE truck-mounted or attached to a crane (Figure 48). Hydraulic drilling rigs are equipped with hydraulic motors that can be Construction, inspection, post-construction integrity testing, moved up and down the mast and are usually truck or crawler and load testing of drilled shafts are related directly to de- mounted. Smaller hydraulic units can be mounted on an exca- sign and performance. These activities are carried out in the vator. Hydraulic drilling rigs with significantly increased field and depend on the skill and experience of contractors, power have appeared in the North American market in recent technicians, inspection personnel, and engineers. In this years. Drilling in rock, especially hard rock, generally requires chapter an overview is presented of construction methods for machines with more power than for drilling in soil. Equipment rock sockets. Methods for load testing of rock-socketed with higher torque ratings and additional power has given shafts are reviewed, including several innovative methods more contractors the capability to install rock-socketed shafts that have made load testing more accessible to state trans- than existed previously. This is a positive development for the portation agencies. Illustrative examples demonstrate how U.S. market in that it promotes competition and expands the load testing can contribute to the economical design of rock base of experienced contractors for rock-socket construction. sockets. Constructability issues identified by the survey ques- tionnaire are discussed, and practices that can lead to quality Equipment developed in Europe and now being used by construction are identified. Current practice for inspection some North American contractors uses hydraulic rams con- and quality assurance methods for rock-socketed shafts are figured to rotate or oscillate (rotate back and forth) a steel also reviewed and discussed. Finally, special geologic condi- casing into the ground (Figure 49). Soil or rock is excavated tions that pose unique challenges for design and construction from inside the casing using a hammergrab, a percussion tool of rock sockets are described, and approaches for using rock that breaks and removes soil or rock. In most cases the rota- sockets successfully in such environments are identified. tor or oscillator is bolted to a crane for stability under the large torque that must be developed. The crane can also pro- vide hydraulic power to operate the rams that turn the casing. CONSTRUCTION OF ROCK SOCKETS Rotators have the capability to cut through high-strength The art and science of drilled shaft construction are as im- rock in the range of 100150 MPa (1522 ksi) depending on portant to the success of a bridge foundation project as are the degree of fracturing (J. Roe, Malcolm Drilling, personal the analytical methods used to design the shafts. Construc- communication, Oct. 3, 2005). A 3-m-diameter oscillator such tion of shafts in rock can be some of the most challenging and as the one shown in Figure 49 is generally limited to cutting may require special expertise and equipment. Experience through weaker rock with strength less than 100 MPa. The demonstrates that the key components of success are: (1) ad- lead casing on the oscillator must have teeth set in opposite equate knowledge of the subsurface conditions, for both directions to cut back and forth. Both methods are efficient design and construction; (2) a competent contractor with the in penetrating large cobbles and boulders, a situation com- proper equipment to do the job; and (3) a design that takes mon to glacial till deposits and cemented sands and gravels. into account the constructability of rock sockets for the par- ticular job conditions. Publications that cover drilled shaft construction methods include Greer and Gardner (1986) and Rock Cutting Tools O'Neill and Reese (1999). Aspects of construction that are related to rock sockets are reviewed herein. Selecting the proper cutting tool depends on many variables, including rock mass properties (strength, hardness, and struc- ture), type of drilling machine, socket depth and diameter, con- Drilling Methods and Equipment dition and cost of the tools, operator skill, previous experience in similar conditions, and judgment. There are no absolute Most rock-socketed shafts are excavated using rotary drilling rules and different contractors may take a completely different equipment. A rotary drill may be mechanically driven or use approach when faced with similar conditions. New tools and hydraulic motors. Mechanically driven rigs deliver power to a innovations are constantly being introduced. Following is a stationary rotary table that rotates a kelly bar to which exca- summary of some of the most common cutting tools used for vation tools are attached. Mechanically driven rigs can be rock-socket construction.
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72 FIGURE 50 Rock auger with drag bit and bullet-shaped cutting teeth. of cutting and crushing. Rock augers may be stepped or tapered so that the initial penetration into rock requires less torque and crowd, or the socket may be drilled first by FIGURE 48 Intact rock core removed using crane-mounted a smaller diameter tool such as the one shown in Figure 51, rotary drill and core barrel. followed by a larger diameter auger. This releases some of the confinement and causes less wear and tear on the drilling tools. Replacement or reconditioning of rock auger teeth can When relatively stiff soil or weak rock cannot be pene- be a major contractor cost, especially in highly abrasive rock. trated efficiently with typical soil drilling tools (e.g., open helix augers), most contractors will attempt to use a rock Self-rotating cutter bits combine a highly efficient cutting auger. Rock augers are manufactured from thicker metal mechanism with the durability of some conical bits. A rock plate than soil augers and have cutting teeth. The teeth may auger with self-rotating cutters, for excavating the face of the be of the drag bit type, which are effective in cutting rock but socket, and conical bits directed outward is shown in Figure wear rapidly and must be replaced frequently. As a rule of 52. A contractor using this auger reported penetration rates thumb, these types of teeth are limited to cutting rock of com- two to three times higher than with conventional rock augers pressive strength up to approximately 48 MPa (7,000 psi), at and in very hard (100 MPa or 15,000 psi) rock. which point they dull quickly. Conical-shaped teeth made of tungsten carbide or other alloys depend on crushing the rock At some combination of rock strength and socket diame- and are more durable than drag bits, but require considerable ter rock augers are no longer cost-effective. One contractor downward force (crowd) to be effective. Figure 50 shows a rock auger with both types of teeth, to exploit both mechanisms FIGURE 49 Casing oscillator and hammergrab tool. FIGURE 51 Small diameter rock auger for creating a pilot hole.
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73 FIGURE 54 Welding roller bits on a 4-m-diameter double- FIGURE 52 Rock auger with conical teeth and rotating cutters walled core barrel. (Courtesy : V. Jue, Champion Equipment, Inc.). chisel, a metal tool that is wedged between the barrel and the interviewed for this study stated that, for socket diameters up rock to fracture the core. The core will usually jam into the to approximately 1.8 m and rock strength up to 70 MPa barrel and can be lifted out of the hole and then removed by (10,000 psi), initial cost estimates are based on the assump- hammering the suspended barrel (see Figure 48). If the rock is tion that rock augers will be used. If the combination of highly fractured, the core barrel may be removed, followed by socket diameter and rock strength exceeds those values, the excavation of the fractured rock from the hole. For deep sockets job is bid on the assumption that rock will be cored. Of or for harder rock, double wall core barrels may be used. The course, these rule-of-thumb criteria are subject to change on outer barrel is set with teeth, typically roller bits (Figure 54), the basis of rock mass characteristics, experience, etc., and while the core is forced into the inner barrel. Compressed air will vary between contractors. Use of a single parameter, is circulated between the barrels to remove cuttings. such as uniaxial compressive strength of rock, does not cap- ture all of the variables that determine penetration rates for a For very high strength rock (qu 100 MPa) there are few given set of conditions. tools that will excavate efficiently. In these rocks, however, even a small penetration can provide high axial, and in some Coring is a widely used method when rock augers are no cases lateral, resistance. A shot barrel, in which hard steel longer feasible. The basic concept is that coring reduces the shot is fed into the annular space between the double walls volume of rock that is actually cut by the teeth. A simple con- of the core barrel, may work in such conditions. Grinding ac- figuration consists of a single cylindrical barrel with cutting tion of the shot excavates the rock and water is circulated for teeth at the bottom edge (Figure 53). The teeth cut a clearance cooling the shot. on the inside and outside of the barrel that is sufficient for re- moving cuttings and extraction of the core barrel. The core Excavation rates with core barrels are typically slow. Al- may break off at a discontinuity or it may require use of a rock though coring may be cost-effective because of the founda- tion performance benefits achieved, careful attention should be given to avoiding overly conservative designs that signif- icantly increase the cost of drilled shafts made by unneces- sary coring into rock. Hard rock can also be excavated using downhole hammer bits. The tool shown in Figure 55 has an array of button-bit hammers (called a cluster drill) operated independently by compressed air. Air pressure also lifts the cuttings which are collected in a calyx basket. On the tool shown in Figure 55, some of the bits can be rotated outward to create a larger diameter socket (under reaming) than the casing, and then retracted to remove the bit. This allows a casing to be installed directly behind the bit during drilling. Downhole hammers and cluster drills are generally expensive and require large air com- pressors to operate. Most contractors will rent this equipment FIGURE 53 Typical single wall core barrel. when needed, which is only cost-effective in very hard rock.